Methods and apparatus for optically measuring polarization rotation of optical wavefronts using rare earth iron garnets

ABSTRACT

Described are the design of a rare earth iron garnet sensor element, optical methods of interrogating the sensor element, methods of coupling the optical sensor element to a waveguide, and an optical and electrical processing system for monitoring the polarization rotation of a linearly polarized wavefront undergoing external modulation due to magnetic field or electrical current fluctuation. The sensor element uses the Faraday effect, an intrinsic property of certain rare-earth iron garnet materials, to rotate the polarization state of light in the presence of a magnetic field. The sensor element may be coated with a thin-film mirror to effectively double the optical path length, providing twice the sensitivity for a given field strength or temperature change. A semiconductor sensor system using a rare earth iron garnet sensor element is described.

This application claims the benefit of U.S. patent application Ser No.60/105,126, filed Oct. 21, 1998, the entire disclosure of which ishereby incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates in general to sensors which use rare-earth irongarnets as sensor elements, and in particular to sensors which use suchgarnets to rotate linearly polarized light for measurement of magneticfields, electrical current, or temperature fluctuations.

2. Related Art

The rapid technology expansion in the military and commercial industrieshas dramatically increased the need for small, highly robust methods ofmonitoring parameters such as magnetic fields, electrical current flow,and temperature. Fiber optic sensors capable of monitoring theseparameters have been under research and development for the better partof 30 years and have recently seen a significant improvement inperformance as new advances in materials and manufacturing are made.Unfortunately, the practical implementation aspects of many of the priorinventions leave much to be desired in terms of ease of use.

The need to monitor magnetic fields and electrical current is enormous.Power companies are concerned with power losses all the way from thegeneration point to the final destination—the consumer. Additionally,power companies bill consumers based upon consumption; unfortunately,they still use monitoring technology that predates most of modernmemory. Loss of the capability to monitor current flow results inpotentially large losses in revenue. Power electronic systemmanufacturers, those who are responsible for converting standardelectrical mains to varying levels of AC and DC, are constantly facedwith limitations in sensor and control technology in the implementationof their designs. Their goals of increased efficiency (lower heat),smaller designs for a given power density, and increases in switchingfrequency are pushing the limits of conventional current and magneticsensing technology. Coupled with this industry is an olderindustry—motor controls. Personnel involved with motor controls areconstantly seeking better methods of increasing output efficiencythrough optimization of run-time parameters—all of which are derivedfrom magnetic and current sensors. Physical limitations in existingsensor capabilities are restricting large advances in hybrid-motordevelopment, which has had a measurable impact on the development ofautomotive hybrid engines.

Optical sensors are poised to revolutionize the sensor industry throughtheir intrinsic advantages. In many instances the bandwidth of opticalsensors is limited only by signal processing constraints—not the sensormaterial as is the case with many conventional sensors. Optical sensorsare often immune to electromagnetic interference (EMI) noise; hence,they do not require specialized shielding in high-noise environments.This results in a smaller transducer cable with significantly lessweight added to the entire assembly, an added benefit for industrial andaerospace/aircraft applications. It also removes the need for localizedsignal conditioning equipment to be positioned close to the monitoringpoint, resulting in potential savings in overall systems cost. Becauseoptical sensors are typically small devices as compared to theirconventional sensor counterparts, they have the potential to fit intosmaller areas or be integrated into existing designs with littlemodification. The dielectric nature of optical fiber gives it anintrinsic isolation in signal measurement in high-voltage or -currentapplications, which is a considerable benefit to the electrical powerand power semiconductor industry. Additionally, optical sensors areinert, which allows their use in potentially explosive environments.Finally, optical sensors can be remotely positioned from the signalprocessing equipment, an advantage that has tremendous benefits for theaerospace, aircraft, and automotive industries.

Certain materials change the polarization state of incident light in thepresence of a magnetic field. This property, known as the Faradayeffect, is widely used in the fiber optic telecommunications field,specifically to prevent reflected light energy from coupling back into alight source and changing source parameters such as frequency or poweroutput. In sensor systems that exploit the Faraday effect, a sensorassembly is placed into a magnetic field. By monitoring the rotation ofthe incident polarization state, a direct measurement of the magneticfield intensity can be inferred.

Optical fiber is one material that exhibits a small Faraday effect.Based upon this, many inventions have been disclosed which measure theamount of current flowing through a conductor. By wrapping multipleturns of optical fiber around the conductor and applying Ampere's Law(increasing the path length), the amount of current can be directlymeasured. Unfortunately, this sensor method is often impractical in manyapplications because it is not feasible to interrupt power bydisconnecting the conductor, installing the fiber coil assembly, thenreconnecting the conductor.

Certain crystalline materials, known as rare-earth iron garnets (REIGs),exhibit a much larger Faraday rotation per unit magnetic flux densitythan their fiber optic waveguide counterparts. These crystals aresynthetic and typically (but not exclusively) have a general formulaequation

R_(3−x)Bi_(x)Fe_(5−x)A_(y)O₁₂

where R is from the family of elements comprising Y, La, Ce, Pt, Nd, Sm,Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, or Lu, A is from the family of elementscomprising Ga, Sc, Al, or In, and 0.3<=x<=2.0 and 0.0<=y<=1.0. A numberof methods exist to grow these materials and range from the flux methodto the Czochralski method to liquid-phase epitaxial growth (LPE). Thepreferred method of growing the crystals is subjective and is somewhatdependent upon the operating wavelength of interest; no preferred methodis implied or required for the present invention as sensor materialsfrom both LPE and flux methods were implemented.

Sensor housings which use a REIG material have historically beendesigned in transmission mode—linearly polarized light enters one sideof the crystal, travels the length of the crystal, and then exits thedistant endface. Optics are required at the distant endface to eitherdirectly quantify the amount of rotation, or to couple the energy backinto a waveguide so that it can be remotely optically processed.Although this configuration can work in many applications, axialalignment of the optical components during manufacturing renders theseconfigurations difficult to mass-produce. This topology also introducessize constraints in the fiber optic embodiment of the invention—theseparate lead-in and lead-out fiber requirement increases the overallsize of the sensor probe due to increased bulk optics, often making themonitoring of magnetic flux in applications such as switched-reluctancesystems very difficult.

U.S. Pat. No. 4,563,639 (1986) to Langeac discloses a temperature and/orelectrical intensity measuring apparatus based upon the Faraday effect.The sensor is an optical fiber wound in the form of a solenoid and isconnected to a polarized light source as well as a beamsplitter andphotodiodes. Through selective twisting of fiber, problems due tobirefringence in the lead-in and lead-out fibers are overcome.

U.S. Pat. No. 5,463,316 (1995) and U.S. Pat. No. 5,493,222 (1996) toShirai et al. discloses a reflection type magneto-optic sensor head. Thehead assembly specifically uses an integrated polarizer, a Faradayrotator comprised of a (111) bismuth-substituted iron garnet singlecrystal film, and a reflecting film, all of unitary construction suchthat each are in contact with the other. Also disclosed is anotherembodiment of this basic concept using rectangular prisms. The lightlaunch/recovery system uses a laser light source, an input collimatinglens, half mirror, multimode polymer-clad optical fiber, an outputcollimating lens, and a single photodiode. No discussion with respect tosignal processing is made.

U.S. Pat. No. 5,483,161 (1996) to Deeter et al. discloses a magneticfield sensor utilizing high-permeability magnetic flux concentratorswith a high-permeability magneto-optic sensing element to increasemeasurement sensitivity. The sensor is used in transmission mode(separate transmit and receive fibers) and utilizes convex lenses tocollimate the polarized wavefront.

OBJECTS AND SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide an improved sensorfor measuring polarization rotation of optical wavefronts.

It is a further object of the invention to provide a fiber opticreflectance-type system that does not require a separate optical path torecover the measurand.

In a preferred embodiment, the invention provides sensors that userare-earth iron garnets as sensor elements to measure magnetic fields,electrical current, or temperature fluctuations. The invention may beprovided in the form of a bulk-optical system preferably aligned in theorder of a laser light source, a polarizer, a chopper, a first-surfacemirror, an anti-reflection coating, a Faraday rotator material, areflecting film, a polarization beam splitter, and two low-noisedetectors. The invention may also be embodied in a fiber optic systempreferably comprising a light source pigtailed to optical fiber, an optoisolator, a fiber optic coupler, optical fiber, a grin lens, ananti-reflection coating, a Faraday rotator material, a substrate, areflecting film, a polarization beam splitter, and two low noisedetectors. A further embodiment provides a semiconductor sensor systemusing a rare earth iron garnet sensor element.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features, and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments as illustrated in the accompanyingdrawings, in which reference characters refer to the same partsthroughout the various views. The drawings are not necessarily to scale,emphasis instead being placed upon illustrating principles of theinvention.

FIG. 1 shows the schematic of the fiber optic magneto-optic sensorassembly, which is based upon the Faraday effect of materials placed ina magnetic field;

FIG. 2 shows a schematic of the fiber optic magneto-optic sensorassembly used for low-profile measurements of current or magnetic fluxdensity;

FIG. 3 shows the complete fiber optic system capable of processing theoptical signal and applying it to the signal processing electronics;

FIG. 4 shows the functional block diagram of the driver section for theLED or laser assembly;

FIG. 5 shows the operation of the bulk optic sensor element;

FIG. 6 shows the complete optical path of the bulk-optical systemcapable of characterizing the sensor element in terms of sensitivity andfrequency response; and

FIG. 7 shows the basic signal processing electronics to recover a signalproportional to the polarization state of rotation.

FIG. 8 shows a concept of the sensor element integrated with a simplesemiconductor to provide on-chip monitoring of current, using a singlewaveguide.

FIG. 9 shows a concept of the sensor element integrated with a simplesemiconductor to provide on-chip monitoring of current, using twowaveguide assemblies.

DETAILED DESCRIPTION

FIG. 1 shows the schematic of a magneto-optic sensor assembly inaccordance with a first, fiber optic, embodiment of the invention. Thesensor of the invention uses the Faraday effect of materials placed in amagnetic field. The sensor 17 shown in

FIG. 1 begins with linearly polarized light of known orientation 2propagating down polarization maintaining single mode optical fiber(PM-SMOF) 3. The propagating wavefront encounters a quarter wavelengthpitch graded index (GRIN) lens 5 after passing through the fiber/GRINinterface 4. This interface is comprised of an optical epoxy that istransparent to the primary propagating wavelength and the preferredrefractive index of the epoxy is given by geometric mean of theinterface refractive indices, or

{square root over (R _(f) R _(G))}

where R_(f) is the refractive index of the fiber core and R_(G) is therefractive index of the GRIN lens. Representative values of R_(f), R_(G)could be 1.45 and 1.61 respectively, at the wavelength of interest.After passing through the GRIN lens the optical wavefront is collimated15. This wavefront encounters another epoxy interface whose preferredrefractive index is given by

{square root over (R _(G) R _(AR))}

where R_(G) is the refractive index of the GRIN lens and R_(AR) is therefractive index of the antireflection (AR) coating 8, or, in the casewhere this coating is omitted, the refractive index of the REIG crystal10. Again, representative values of R_(G), R_(AR), and R_(REIG)(R_(REIG) is the refractive index of the REIG crystal in a preferreddirection) are 1.61, 1.31, and 2.1 respectively. The wavefrontpropagates through the REIG crystal, and in the presence of the magneticfield H 1 with some component parallel to the direction of wavefrontpropagation, will undergo a rotation θ (theta) dependent upon crystalparameters and the single direction optical path length L 11. If theREIG crystal 10 has been epitaxially grown on a transparent non-magneticgarnet substrate 12, such as Gd₃Ga₅O₁₂, (also known as GGG) then thewavefront will propagate through the GGG substrate and will reflect fromthe dielectric or metallic thin-film mirror 13, which could be comprisedof aluminum.

After reflecting from the thin-film mirror 13 the wavefront traces theincident path. After traveling through the REIG crystal 10 the wavefrontundergoes yet another rotation θ (theta) due to the non-reciprocity ofthe REIG crystal. The collimated wavefront is collected in GRIN lens 5and is refocused onto the core of the fiber 3. This final wavefront 16,still linearly polarized, is offset from the incident polarization stateby an amount 2θ (2*theta) due to the doubling of the optical path lengthproduced by the thin film mirror 13.

Additional design constraints exist in sensor 17 as shown in FIG. 1. Themode field diameter of SMOF in the visible wavelength range isapproximately 5.5 μm (10e−6m) or less. As wavelength increases so doesthe core size of the fiber, reaching 9 μm at 1300 nm and 11 μm at 1550nm. This alone has a tremendous impact upon the amount of energy placedinto the system as well as recovered, and use of a GRIN lens 5 increasesthe amount of power recovered nearly by 4 dB. Additionally, as modefield size decreases, the individual probing of magnetic domains occursin a system not using a GRIN lens, resulting in localized “hotspots/deadspots” in rotational values. The use of a GRIN lens 5 to couple energy 2from the SMOF fiber 3 into the REIG crystal 10 results in the opticalwavefront being spread over a much larger surface area of the actualsensor material 10, hence giving rise to an “average domain effect”. Theresult is enhanced stability and repeatability in measurements. Althoughthe GRIN can be omitted, there is a significant penalty in the amount ofreturned power to the detection system which has negative implicationson signal to noise ratio.

As previously stated, the AR coating 8 may or may be provided. Ifimplemented, the thickness t 9 of the AR coating is given by the generalformula t=N * λ/4, where Nε{1,3,5, . . .} and λ is the primarywavelength of the propagating wavefront. Factors governing whether ornot the AR coating 8 is implemented are the value of λ selected andproperties characterizing the REIG crystal which govern overall systemperformance.

Shown on sensor 17 in FIG. 1 is an angle α (alpha) 6 that is measuredbetween the endface of the GRIN lens and the incident interface ofeither the AR coating 8 or the REIG crystal 10. Experimentally it hasbeen shown that this angle polished onto the GRIN such that 0<=α<=11degrees can result in significant decrease in reflected optical energyfrom the AR coating 8 or the REIG crystal 10. This decrease incontinuous optical energy manifests itself as a decrease in thebackground DC, resulting in a significant improvement of systemsensitivity. The value of α (alpha) is determined by the overall desiredSNR of the system and mode of operation (whether the system will be usedin AC and/or DC monitoring applications).

Shown on sensor 17 in FIG. 1 is a non-magnetic garnet substrate 12, suchas GGG. The presence of this material is dependent upon the choice ofREIG material 10 and the method used to fabricate the REIG crystal. Forexample, REIGs grown through liquid phase epitaxial (LPE) growth methodsuse the GGG as the seed; hence GGG will be included in the sensorconfiguration. Contrasting, other REIGs grown using a flux growthtechnique require no GGG substrate; hence, a pure REIG crystal can beimplemented in the sensor configuration. Examples of this areBi-substituted yttrium iron garnet (Bi-YIG) crystals grown in a LPEfurnace and pure YIG crystals produced using flux techniques. Both arewell documented in the literature and samples of each have beenimplemented. Other methods available to grow REIG materials withmagneto-optical properties are also metal-oxide chemical vapordeposition (MOCVD) and the use of sol-gel processes. No preferred methodis implied with respect to this invention.

The sensor 99 shown in FIG. 2 shows the basic embodiment of sensor 17 ofFIG. 1 enclosed within a silica hollow-core tube 85 such as that asmanufactured by Polymicro Technologies. Operation starts with linearlypolarized light of known orientation 93 propagating down PM-SMOF 81. Thepropagating wavefront encounters a quarter wavelength pitch GRIN lens 86after passing through the fiber/GRIN interface 84. This interface iscomprised of an optical epoxy that is transparent to the primarypropagating wavelength and the preferred refractive index of the epoxyis given by geometric mean of the interface refractive indices, or

{square root over (R _(f) R _(G))}

where R_(f) is the refractive index of the fiber core and R_(G) is therefractive index of the GRIN lens. Representative values of R_(f), R_(G)could be 1.45 and 1.61 respectively, at the wavelength of interest.After passing through the GRIN lens the optical wavefront is collimated.This wavefront encounters another epoxy interface whose preferredrefractive index is given by

{square root over (R _(G) R _(AR))}

where R_(G) is the refractive index of the GRIN lens and R_(AR) is therefractive index of the AR coating 88, or, in the case where thiscoating is omitted, the refractive index of the REIG crystal 89. Again,representative values of R_(G), R_(AR), and R_(REIG) (R_(REIG) is therefractive index of the REIG crystal in a preferred direction) are 1.61,1.31, and 2.1 respectively. The wavefront propagates through the REIGcrystal, and in the presence of the magnetic field H 94 with somecomponent parallel to the direction of wavefront propagation, willundergo a rotation θ (theta) dependent upon crystal parameters and thesingle direction optical path length. After the wavefront has propagatedthrough the REIG crystal 89 it will reflect from the dielectric ormetallic thin-film mirror 90, which could be comprised of aluminum.

After reflecting from the thin-film mirror 90 the wavefront traces theincident path. After traveling through the REIG crystal 89 the wavefrontundergoes yet another rotation θ (theta) due to the non-reciprocity ofthe REIG crystal. The collimated wavefront is collected in GRIN lens 86and is refocused onto the core of the fiber 81. This final wavefront 92,still linearly polarized, is offset from the incident polarization state93 by an amount 2θ (2* theta) due to the doubling of the optical pathlength produced by the thin film mirror 90.

As previously stated the AR coating 88 may or may not exist. Ifimplemented, the thickness t of the AR coating is given by the generalformula t=N*λ/4, where N ε{1,3,5, . . . } and λ is the primarywavelength of the propagating wavefront. Factors governing whether ornot the AR coating 88 is implemented are the value of λ selected andproperties characterizing the REIG crystal which govern overall systemperformance.

As referred in FIG. 2 the size of the GRIN lens 86 and the REIG sensorelement 88,89,90 are shown such that the endface of the GRIN lens 89 issmaller than that of the REIG element 88,89,90. This is not a constraintof the sensor design 99; optimally the GRIN lens would be the samediameter as the REIG sensor element 88,89,90 so that maximal volume ofthe element is used as the sensing media. In FIG. 2 the entire assembly,which physically consists of components (in optical incidence order)PM-SMOF 81, GRIN lens 86, and REIG sensor element 88,89,90, can bebonded into a cylindrical housing, either metallic or non-metallic, toprovide stability for the sensor head and to provide a degree ofenvironmental protection. One method to secure the sensor assembly tothe housing is through epoxy fillets 82 and 89, which may or may notencircle the fiber 81 and REIG sensor element 88,89,90 360 degrees. If ametallic housing is utilized then care must be given to the permeabilityof the material, for this could redirect flux lines around the sensorassembly rather than through the REIG crystal element. A thin-walledsilica (SiO₂) hollow-core tube 85, possibly 1 mm in diameter, issufficient to demonstrate this concept and remove any chance thatmagnetic lines of flux are being redirected. Potentially, redesign ofthe concept of Deeter et al. (U.S. Pat. No. 5,483,161) in place ofhollow-core housing 85 could allow for greater sensitivity of the sensorhead. This potential design modification does not alter the basicoperation of the sensor 99.

FIG. 3 presents the complete fiber optic system 35 to support magneticfield, electrical current, or temperature measurements. A LED or lasersource 20 pigtailed to single mode optical fiber (SMOF) 24 is fusionspliced 21 to SMOF from the input side of a Faraday isolator 22.Depending upon the pigtail configuration, the LED/laser 20 may or maynot use a focusing element. The output of the Faraday isolator 22 is alinearly polarized wavefront 2 of known polarization orientationtraveling in either SMOF or PM-SMOF depending upon isolatormanufacturer. The output fiber from the Faraday isolator 22 is fusionspliced 21 to the input arm of a polarization maintaining single modeoptical 2×2 coupler (PM-SMOC) 23 such that the fast and slow axis' ofthe PM-SMOC are oriented at the preferred angle of 45 degrees withrespect to the orientation of the wavefront 2. The polarized wavefronttravels through the PM-SMOC 23 and is coupled equally into each of theoutput arms. One of the output arms of the PM-SMOC 23 is fusion spliced21 to standard PM-SMOF 3 and is coupled into the sensor assembly 17 aspreviously described. The other output arm of the PM-SMOC 23 is coupledto a photodiode 25, which provides drive information to the laser/LEDdriver circuitry. Note that photodiode 25 can be omitted and not changethe overall system operation.

The output of the sensor assembly 17 is a rotated polarization state oflight 16 proportional to the intensity of the magnetic field in parallelwith the incident beam. This wavefront travels back through the PM-SMOC23 (from right to left as shown in FIG. 3) and splits into two paths:one that travels back towards the source 20 and the other towards theremaining analyzer 29 and photodiodes 31,33. With respect to the formerpath this wavefront is phase shifted 45 degrees plus an amountproportional to the strength of the magnetic field measured by thesensor 17. After propagating through the isolator 22 the wavefront willundergo an additional 45 degrees rotation, causing the overall rotationto be 90 degrees ± the sensor rotation 2θ (theta). This optical energywill couple back into the laser/LED source 20, but relative strength ofthis signal is approximately equivalent to the forward power multipliedby the sine of the rotation 2θ less 9 dB. In other words, the backwardcoupled energy into the source can be described by

P_(r)≈P_(i)*sin(2θ)−9 dB

where P_(r) is the amount of reflected power incident upon the source20, P_(i) is the amount of power originally output by the source andcoupled into the launch fiber, and θ (theta) is the single-path rotationdue to an external magnetic field. This value, although significantenough to effect changes in the quantum efficiency of the source cavity,can be compensated and somewhat minimized in electronics using a powermonitoring photodiode 25 in addition to an intrinsic back-facet monitorphotodiode 122 (FIG. 4).

With respect to the signal traveling in the lower left arm of FIG. 3,the wavefront 16, now reduced by at least 3 dB due to the properties ofthe PM-SMOC 23, is coupled from the coupler arm to a GRIN lens 27through interface 26. As in the sensor arm, the two are coupled via anoptical epoxy with a refractive index that is the geometric mean betweenthe refractive index of the core of the PM-SMOC arm and the refractiveindex of the GRIN lens 27. The output of the GRIN 27 is a collimatedwavefront that is now incident on an analyzer 29, in this case anorthogonal polarization beamsplitter (PBS) with a 500:1 extinctionratio. In the preferred embodiment the GRIN is attached to the PBS 29with an optical epoxy with a refractive index that is the geometric meanbetween the refractive index of the GRIN 27 and the PBS 29. The PBS ispositioned such that it provides two outputs, 30 and 32, to twolow-noise photodiodes 31 and 33 respectively. Each output is orthogonalto the other, that is, the relative intensity of each output isindependent of the other such that the two intensities describe therotational state of the polarization vector.

Shown in FIG. 4 is a block diagram of the major functions of theLED/laser drive electronics 139. This configuration helps to reducenoise, which modulates the LED/laser and consequently, is detected inthe signal processing function 119 of FIG. 7. Operation begins shortlyafter turn-on through the precision, low noise reference voltage source120. This source is one input to an operational amplifier (opamp)subsystem 127. The other input is produced from the opto-electricalchain consisting of forward power-monitoring photodiode 25, a low-noisetransimpedance (I-V) amplifier subsystem 123, and an 8^(th)-orderButterworth low-pass filter subsystem 125. At turn-on the input fromthis second chain is nearly zero, hence the error between the inputs of127 is large. This large voltage command is sent to one input of anotheropamp subsystem 128. As in the previous opto-electronic conversion chainthe second input to opanp 128 is comprised of a back-fact monitorphotodiode 122, a low-noise I-V amplifier subsystem 124, and an 8CorderButterworth low-pass filter subsystem 126. At turn-on the input fromthis second chain is nearly zero, hence the error between the inputs of128 is large. This large error command is sent to a low-noisetransconductance (V-I) amplifier subsystem 129 which converts thecommand voltage from the previous stage 128 into a current command. Asoptical power within the system 35 (FIG. 3) rises the two I-V conversionchains become non-zero, reducing their respective error commands to theinputs of opamp subsystems 127, 128, and hence to V-I converter 129. Thecurrent command then biases the LED/laser 20 and the system eventuallystabilizes to a predetermined optical output level.

The second opto-electronic conversion chain in FIG. 4 can be optionaldepending upon whether or not the LED/laser 20 has a back-facetphotodiode monitor available. In the event that the there is noprovision for this opamp 128 can be bypassed and tied directly to theV-I amplifier 129. The impact of doing this is a potential noiseincrease in the system.

Low-pass filters 125 and 126 can be set to provide signal responsebandwidths to approximately 220 kHz, but typically are set lower tominimize noise within the system. Note that these filters have minimalimpact upon the detection system bandwidth; they are provided solely toremove random intensity noise and electronic noise in the powergeneration system 139. These filters 125,126 control the loop responserate to sudden increases or decreases in noise power within thegeneration system 139 and are tailored depending upon the specificcharacteristics of the LED/laser source 20.

Shown in FIG. 5 is the sensor element 79, which is the foundation of thesensing mechanism. The sensing element 79 is comprised of at least twolayers, and potentially four, depending upon the intended application ofthe element. Linearly polarized light of known orientation 61 isincident in free space 62 at a nearly normal angle to the endface ofelement 79. This wavefront penetrates an AR layer 63 of thickness t 69,or, in the case where this coating is omitted, the REIG crystal 64. Asthe wavefront propagates through the REIG crystal and in the presence ofthe magnetic field H 71 with some component parallel to the direction ofthe wavefront propagation, will undergo a rotation θ (theta) dependentupon crystal parameters and the single direction optical path length L70. If the REIG crystal 64 has been epitaxially grown on a transparentnon-magnetic garnet substrate 65, such as GGG, then the wavefront willpropagate through the GGG substrate and will reflect from the dielectricor metallic thin-film mirror 66, which could be comprised of aluminum.

After reflecting from the thin-film mirror 66 the wavefront nearlytraces a normal path, but slightly off axis due to the misalignment ofthe incident wavefront 61. After traveling through the REIG crystal 64,the wavefront undergoes yet another rotation θ (theta) due to thenon-reciprocity of the REIG crystal. This final wavefront 67, stilllinearly polarized, is offset from the incident polarization state by anamount 2θ (2* theta) due to the doubling of the optical path lengthproduced by the thin film mirror 66.

The AR coating 63 may or may not exist. If implemented, the thickness t69 of the AR coating is given by the general formula t=N* λ/4, whereNε{1,3,5, . . . } and λ is the primary wavelength of the propagatingwavefront. Factors governing whether or not the AR coating 69 isimplemented are the value of λ selected and properties characterizingthe REIG crystal which govern overall system performance. FIG. 6presents the complete bulk-optical system 59 to support magnetic field,electrical current, or temperature measurement. A helium-neon (HeNe)laser source launches highly coherent light through polarizer 42, whichassigns a known polarization state 43 to the optical wavefront. Thiswavefront travels through an optical chopper 44, which is used tomodulate the laser source 41 to enable low-level signal measurements.The modulated beam strikes first-surface mirror 45 at a location 46 suchthat the normal to the mirror 45 and the normal to the sensor element 79are not parallel. This directs the optical wavefront off normal axis tothe sensor element 79. After propagating through the sensor element aspreviously described the wavefront exits the sensor element 79 andstrikes the mirror 45 at a location 48. This directs the modulatedoptical wavefront 49 through free space towards a PBS 50 aligned in thepath of the wavefront. The PBS 50 splits the wavefront into two beams ofindependent intensity; the intensity is a function of the polarizationangle of the wavefront 49. Two low-noise photodetectors 52,54 arepositioned such that the optical outputs 51 and 53 from the PBS 50 areincident on their active surfaces.

Shown in FIG. 7 is the analog electronics implemented to quantify therotational value of the polarization state from some initial startingvalue. The electronic opto-electronic system is a preliminary part ofthe overall signal processing system, and represents the foundation of amuch larger algorithm that is required to use either of the systems in acontinuous, long-term monitoring application.

Operation of the opto-electronic front end 119 begins with twoindependent intensities arriving at photodiodes 31 and 33. These signalsare the decomposed orthogonal components of the polarization vector suchthat a rough estimation of the polarization state can be determined by

β=Arc tan(p/s)

where β is the value of rotation, p is one of the intensity components,and s is the other intensity component. The output of each photodiode isprocessed by a low-noise transimpedance (I-V) operational amplifiersubsystem 103 and 105 to provide a voltage proportional to the currentinput. For the sake of this discussion, the output of I-V converter 103is termed the p component and the output of I-V converter 105 the scomponent 106. These are purely arbitrary assignments.

Although the p/s relationship in the argument above is valid, concernswith common-mode intensity fluctuations of the LED/laser source 20 ofFIG. 3, in addition to the limited dynamic range of the argument (if sgoes to 0 then loss of directionality occurs) forces a more robustalgorithm. Although a considerable number of potential methods exist tomeasure the polarization state rotation, a modification of the method ofMansuripur et al. (Applied Optics, 29 (9), Mar. 20, 1990) was selecteddue to its ease of implementation. Correspondingly, the method selecteduses a difference/sum relationship that is given by

$\frac{p - s}{p + s}$

where p and s were previously defined.

To provide this value, the remaining part of FIG. 7 details the majorfunctional sections. Signal s 106 is inverted by unity gain invertingamplifier 108 and is applied to summation amplifier 107. The other inputto this amplifier 107 is a p input; hence, the output produced is p−s109. Signal s is also applied to summation amplifier 110 whose otherinput is signal p; the output of this amplifier 110 is p+s 111. Thesesignal outputs 109, 111 are applied to a logarithmic division amplifier112 to produce the required output 113.

The output of the front end of the signal processing system 119represents a direct measurement of the rotational value of thepolarization state, which could be a function of magnetic flux density,current, or temperature. This value comprises a steady-state DCcomponent, which can be a function of temperature, the steady-statemagnetic field, or backscatter within the system, and an AC component,which contains the dynamic information present within the measuredsignal. Depending upon application, the DC component can be discarded,allowing only for dynamic signals to be monitored, or can be included,to show both AC and DC values. Typically, these output signals are sentto additional signal processing components such as analog-to-digitalconverters, microprocessors, and/or digital-to-analog converters.Configuration is completely application specific.

FIG. 8 shows an embodiment of the invention applied to a powerelectronics device. Shown is a n-channel enhancement mode MOSFET powerelectronics device, but any power electronics device could be used withthis technology. Linearly polarized light of known orientation 150propagating through a planar waveguide 151 and strikes an angledinterface 152, where it is reflected toward through a REIG crystal 153to reflecting dielectric or metalized boundary 154. As in the case of 17(FIG. 1) the light undergoes a rotation of θ (theta) as it travelsthrough the crystal 153 in the presence of a magnetic field 159 withsome portion of the field vector 159 parallel to the direction ofpropagation. This rotated wavefront, having reflected off thin film 154,traces the incident path through the REIG crystal 153 where it againexperiences another rotation θ (theta) in the presence of said magneticfield 159. This wavefront, now being rotated a total of 2θ (2* theta),strikes the boundary 152 of the waveguide and is reflected towards thesource point on the substrate, leaving the waveguide 151 as a rotatedwavefront 155.

The technology presented in the device 169 represents one configurationwhen the magnetic field 159 is oriented perpendicular to the plane ofthe substrate. In the event that the field is horizontal to the plane ofthe substrate the waveguide 151 could be fabricated such that the angledinterface 152 is omitted and the sensor assembly 153,154 could be placedat the end of the waveguide in place of 152. The orientations presentedin 169 are done so to show one particular embodiment in one particularapplication; as the geometry of each semiconductor is different and thewaveguide/sensor structure will change accordingly. No restrictions areimplied to the application of this sensor/waveguide structure for use insemiconductor field monitoring.

The embodiment of device 169 is not exclusive. The same conceptpresented in FIG. 8 is presented as FIG. 9 but omits reflectivesubstrate 154 (FIG. 8) while adding additional waveguide 156. Additionalwaveguide 156 serves as the continuation of the transmission path andcarries the modulated optical wavefront of known polarization state 155to the edge of the semiconductor substrate for subsequent detection andprocessing.

While the invention has been particularly shown and described withreference to a preferred embodiment thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the spirit and scope of theinvention.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. A magneto-optic sensorelement comprising: a crystal substrate; a thin-film reflective surfaceon a first face of said crystal substrate; a rare-earth iron garnetthin-film on a second face of said crystal substrate, said second faceopposing said first face; and, an anti-reflection coating on saidrare-earth iron garnet substrate the anti-reflection coating has athickness t which is within the range: 0<=t<N* .lambda. /4 where lamdarepresents the primary wavelength of an incident polarized wavefront andN is an odd- integer such that 1<=N<=∞.
 2. The magneto-optic sensorelement according to claim 1, wherein said thin film reflective surfacecomprises a dielectric mirror deposited on said crystal substrate. 3.The magneto-optic sensor element according to claim 1, wherein said thinfilm reflective surface comprises a metallic mirror deposited on saidcrystal substrate.
 4. The magneto-optic sensor element according toclaim 1, wherein said crystal substrate is optically transparent withrespect to the wavelength of an incident polarized wavefront.
 5. Themagneto-optic sensor element according to claim 1, wherein saidrare-earth iron garnet thin-film is deposited on said second face ofsaid crystal substrate.
 6. The magneto-optic sensor element according toclaim 1, wherein said rare-earth iron garnet thin-film is grown on saidsecond face of said crystal substrate.
 7. The magneto-optic sensorelement according to claim 1, wherein said anti-reflection coating isdeposited on said rare earth iron garnet substrate.
 8. The magneto-opticsensor element of claim 1, wherein said thin-film reflective surfacedoubles the total single-direction optical path length through saidrare-earth iron garnet thin film and supporting crystal substrate.
 9. Amagneto-optic sensor element comprising: a crystal substrate which isoptically transparent with respect to the wavelength of an incidentpolarized wavefront from a sensor light source, said crystal substratefurther comprising a rare-earth iron garnet crystal; a dielectricthin-film mirror deposited one side of said rare-earth iron garnetcrystal; and, an anti-reflection coating of thickness0<=t<=N*.lambda./4, where lambda is the primary wavelength of anincident polarized wavefront and N is an odd-integer multiple such that1<N<=∞, deposited on the opposite end of the rare-earth iron garnetsubstrate.
 10. The magneto-optic sensor element of claim 9, wherein saidthin-film reflective surface doubles the total single-direction opticalpath length through said rare-earth iron garnet thin film and supportingcrystal substrate.
 11. The magneto-optic sensor element according toclaim 1, further comprising: a graded-index lens for coupling opticalenergy into said crystal substrate.
 12. The magneto-optic sensor elementaccording to claim 11, wherein said graded-index lens is optically tunedto the quarter wavelength of an incident polarized wavefront from asensor light source.
 13. The magneto-optic sensor of claim 12, whereinsaid graded-index lens comprises a quarter-pitch lens at the primarywavelength.
 14. The magneto-optic sensor of claim 11, wherein saidgraded-index lens is in the shape of a right-angled cylinder and isbonded to an anti-reflection side of said crystal substrate via anoptically transparent epoxy.
 15. The magneto-optic sensor element ofclaim 11, wherein said graded-index lens is polished on at least one endto a facet angle of 0<=alpha. <=11 degrees, measured with respect to therotational symmetry axis of the cylinder.
 16. The magneto-optic sensorof claim 11, arranged such that an incident plane polarized light beampropagating through said sensor element travels along the axis of saidgraded-index lens, through said crystal substrate, and strikes saidthin-film mirror at essentially normal incidence.
 17. A magneto-opticsensor probe comprising: a crystal substrate; a thin-film reflectivesurface on a first face of said crystal substrate; a rare-earth irongarnet thin-film on a second face of said crystal substrate, said secondface opposing said first face; a graded-index lens for coupling opticalenergy into said crystal substrate; and, a optical fiber coupled withsaid graded-index lens.
 18. The magneto-optic sensor probe of claim 17,wherein said optical fiber comprises a polarization-maintaining singlemode optical fiber.
 19. The magneto-optic sensor probe of claim 18,further comprising an anti-reflection coating on said rare-earth irongarnet substrate.
 20. The magneto-optic sensor probe of claim 18 whereinsaid polarization maintaining single mode optical fiber is bonded tosaid graded-index lens using optical epoxy.
 21. A fiber optic sensorsystem, comprising: a light source for emitting a light beam; apolarizing means for polarizing said light beam; fiber optic coupler;crystal substrate having a rare-earth iron garnet thin-film on a facethereof; graded index lens assembly optically coupled to said fiberoptic coupler; beamsplitter optically coupled to said graded index lensassembly; and, detector means for converting optical energy intoelectrical energy.
 22. The fiber optic sensor system according to claim21, wherein said polarizing means comprises a Faraday isolator.
 23. Thefiber optic sensor system of claim 22 wherein: said light source iscoupled to said optical fiber via a lensing system such that the outputof said light source is effectively coupled into a core of said opticalfiber; said light source outputs an arbitrary state of polarization;said light source is fusion spliced to an input of said Faradayisolator; said Faraday isolator polarizes and rotates said arbitrarystate of polarization at said isolator input to a known state ofpolarization; said fiber optic coupler comprises a 2×2polarization-maintaining single mode coupler which is arranged such thatthe power ratio between output arms of said coupler is 1:1 and such thatthe known state of polarization produced by said Faraday isolator ismaintained in each output arm; one output arm of said fiber opticcoupler is terminated into a forward power monitoring photodiode;another output arm of said fiber optic coupler is fusion spliced to saidoptical fiber, said optical fiber being optically coupled to saidcrystal substrate; the remaining arm of said fiber optic coupler iscoupled to said graded index lens; the output of the said beamsplitterhaving two independent optical intensities; said signal recoveryphotodiodes being positioned such that the output from the saidbeamsplitter is incident upon their active regions.
 24. The fiber opticsensor system according to claim 21, wherein said fiber optic couplercomprises a 2×2 fiber optic coupler.
 25. The fiber optic sensor systemaccording to claim 21, wherein said detector means comprises two signalrecovery photodiodes.
 26. The fiber optic sensor system according toclaim 21, wherein said light source comprises a laser light source. 27.The fiber optic sensor system according to claim 21, wherein said lightsource comprises an LED light source.
 28. The fiber optic sensor systemaccording to claim 21, further comprising: a forward power monitoringphotodiode into which an output arm of said fiber optic coupler isterminated.
 29. The sensor system of claim 21 wherein said beamsplitteris a polarization beam splitter (PBS) providing at least a 500:1extinction ratio between its two output intensities.
 30. The sensorsystem of claim 28, wherein said forward monitoring power diode providesadditional compensation and noise reduction in the drive electronics ofsaid light source.